Infections caused by bacteria and fungi have always been a major medical problem. The most dangerous of these are systemic infections, i.e., sepsis. Despite progress in their treatment achieved primarily through the use of antibiotherapy and the introduction into medical practice of technologies for prolonged life support of patients in critical condition, we are still failing to keep many patients alive. With the development of medical knowledge and the introduction of newer and newer therapeutic procedures into treatment, the incidence of sepsis is increasing. Lever et al. report that, each year in the U.S., 750,000 people come down with sepsis and it is the cause of more than 215,000 deaths. In the European Union, 146 thousand patients die annually due to severe sepsis. In the UK alone mortality from it ranges from 30 to 50/100,000 a year, which puts sepsis in the forefront of the ten leading causes of death. In developed countries, sepsis develops in 2-4/1,000 live born neonates and it is the main cause of their death. In Poland, there is a lack of accurate epidemiological data, but Zielinski et al. provide information that 967 deaths occurred in 2005 due to sepsis, including 43 deaths of children.
The growing mortality due to sepsis is the result of increasing resistance to antibiotics, the use of invasive treatment methods, and an aging population. Sepsis is the biggest threat to immunocompromised people, especially when they are hospitalized over long periods of time, primarily in intensive care units. Particularly, sepsis affects patients with neoplastic diseases, immunocompromised patients, patients with burns, the elderly, and children.
The most important and most difficult problem in the treatment of bloodstream infections, determining the effectiveness of treatment and, consequently, the costs and duration of hospitalization, is efficacious diagnosis of factors causing the systemic inflammatory response in the course of sepsis. Identification of the etiological agent (microorganism: fungus or bacterium) allows the employment of effective targeted antibiotherapy. The material subjected to diagnostic testing is blood taken from a patient manifesting clinical symptoms of sepsis. Symptoms may include tachycardia, bradycardia, increased or decreased body temperature, drop in blood pressure, etc.
Blood poses the biggest challenges among all biological materials as regards a material for microbiological testing because the microorganisms responsible for infection can be found in blood in very small quantities, or there is only their periodic release into blood.
Nevertheless, the current diagnostic standard are blood cultures performed on special media, ideally in automated culture systems (e.g., BACTEC-BectonDickinson). The advantages of such methods are their simplicity and relatively low costs of testing. Their weakness is that they are time-consuming, taking up to 5 days (to receive results), and have low sensitivity, which causes only 15-20% of the culture to obtain microbial growth. Consequently, in a great majority of cases, the doctor may only apply empirical antibiotherapy due to the lack of achieving growth of microorganisms responsible for the infection. The situation is further exacerbated by the fact of subjecting patients to antibiotherapy before any blood samples are drawn for culture—patients are often treated with antibiotics prior to manifestation of symptoms of sepsis. Blood cultures are very problematic in such a case, due to the fact that the blood now contains antibiotics inhibiting the growth of microorganisms. In order to increase the chance of detecting microbiological agents in blood, attempts are being made to base their detection on serological methods such as the detection of lipopolysaccharide (LPS) of Gram-negative bacteria or fungal galactomannan.
Another molecular target that allows efficient, accurate and quick diagnosis of bloodstream infections are microbial nucleic acids which are etiological agents of infection. Both DNA, as well as RNA, of each organism contains sequences unique to it, constituting a specific “fingerprint”. With the knowledge of these sequences, it is possible to apply molecular biological methods, such as PCR or hybridization, for determining the presence of microorganisms in the blood. Sensitivity of molecular methods considerably exceeds the sensitivity of the culture method. Additionally, the prior use of antibiotic therapy does not influence the test result due to the fact that there is no need for growth of bacteria or fungi in culturing medium, but only detection of their DNA or RNA sequences.